There is currently a great surge of activity in fuel cell research as laboratories across the world seek to take advantage of the energy capacity provided by fuel cells over those of other portable electrochemical power systems. Much of this activity is aimed at high temperature fuel cells and a vital component of such fuel cells must be availability of a high temperature stable proton-permeable membrane. Many groups are exploring high temperature stable polymers for use with systems containing non-volatile bases, e.g. imidizole which becomes the proton carrier. It can be rendered immobile (though locally mobile) by attaching it to a polymer chain and then the proton alone can permeate (though immobility is not essential for membrane function.
Other approaches to the high temperature fuel cell involve the use of single-component or almost-single-component electrolytes that provide a path for protons through the cell. A heavily researched case is the phosphoric acid fuel cell in which the electrolyte is almost pure phosphoric acid and the cathode reaction produces water directly (whereas the cathode reaction in the Bacon cell produces OH− species). The phosphoric acid fuel cell delivers an open circuit voltage of 0.9V at ambient pressure falling to about 0.7 under operating conditions at 170° C. The proton transport mechanism is mainly vehicular in character meaning that the protons are transported as an integral part of a protonated species rather than by a Grotthus type mechanism.
The study of electrical conductance in ionic solutions goes back to the earliest chapters of physical chemistry. It has been overwhelmingly the study of aqueous solutions. The concepts of ionic dissociation and the battles fought to establish the reality of ions were based on observations made on aqueous solutions (1).
The first ionic liquid IL (or ambient temperature molten salt ATMS) reported was ethylammonium nitrate in 1914 [16].
The more recent surge of interest in non-aqueous electrolyte systems (2, 3) has been driven, in part, by the quest for a rechargeable lithium battery. In this respect, the much lower conductivities characteristic of non-aqueous electrolytes has been a serious hurdle (3).
The possibility of obtaining liquids with low vapor pressures by the proton transfer mechanism has been utilized in military programs for some time [29, 30]. In these applications the combination of oxidizing anions with reducing cations in mobile liquids such as hydroxylammonium nitrate (HAN)) containing small, controlled amounts of water) makes possible the controlled redox energy release appropriate for artillery propellants. The ionic liquids formed in these systems seem to have low viscosities, judging by military report data [30] for the partially hydrated practical formulations (that have been included in certain journal publications [31] but no reports of viscosity or conductivity values for the anhydrous ionic liquids have been found.
More recently ionic liquid media have been finding application in various synthetic chemistry processes [32-35] but the great majority of such vaporless liquids have not been of the type described here. On the other hand, the proton transfer from strong acid to base has been utilized recently as a general preparative technique for formation of ionic liquids. Examples reported have had in common the very weakly basic anion bis(trifluoromethanesulfonyl)imide, TFSI [10, 36]. The relation between the protic and aprotic versions of the ionic liquid and in particular the relation between their relative vapor pressures awaits systematic attention.
When the free energy change in the proton transfer process is large the proton may become so firmly localized on the Bronsted base that the Boltzmann probability of reforming an acid molecule becomes negligible at ambient temperatures. In some cases it remains negligible even at temperatures as high as 300K. The salt is then, by most measures, as true a salt as those called “aprotic” ionic liquids (e.g. those formed by —CH3+ transfer rather than proton transfer to the same site). One will see that in fact such liquids can be more ideally ionic than certain individual salts in which there are no such proton transfer sites and in which, accordingly, the positive charge is located in the interior of the ion.
One way of assessing the ionicity of ionic liquids is to use the classification diagram shown in FIG. C1 [14, 37] which is based on the classical Walden rule [21]. The Walden rule relates the ionic mobilities (represented by the equivalent conductivity Λ (Λ=FΣμizi) to the fluidity of the medium through which the ions move. If the liquid can be well represented as an ensemble of independent ions then the Walden plot will correspond closely with the ideal line. Ideally, which means in the absence of any ion-ion interactions, the slope should be unity. The position of the ideal line is established using aqueous KCl solutions at high dilution.
As argued elsewhere {14, 37, 38] a liquid system in which the ions are uniformly distributed with respect to ions of the opposite charge develops a Madelung energy comparable to that of the corresponding crystal. This is demonstrated by the absence of anything unusual about the heats and entropies of fusion of classical ionic systems as would be the case if the Madelung energy were lost on fusion. The vapor pressure of the “good” ionic liquid is then necessarily very low because the Madelung energy as well as the dipole-dipole interaction between ion pairs must be overcome before an ion pair can pass into the vapor state.
Salts formed by proton transfers that are weak will not form liquids with uniform charge distributions hence their Walden plots will fall below the ideal line and their vapor pressures will not be very low. In such cases, the boiling that must occur when the total vapor pressure reaches the external pressure will fall below decomposition temperatures and the vapor will tend to contain molecular species rather than ion pairs. In this work the inventors provide experimental data on a number of binary, solvent-free, Bronsted acid-base systems that will help test these notions.
Not only does presentation of data in the FIG. C1 form allow one to detect the existence of different forms of association of cations with anions, but it also serves to display the presence of abnormally high mobilities of one or other of the charge-carrying species. Excess mobility on the part of protons is a classical subject, and mechanisms that permit its understanding date from the original work of Grotthus, as invoked by Bernal and Fowler [23]. Not so commonly discussed but phenomenologically indistinguishable is the excess conductivity which is found when species that are much larger than protons can slip through the structure via channels that present lower energy barriers than those characterizing the viscous flow process. For these the motion is described by the “fractional Walden rule” Λφα=constant where α<1.
In the log-log plot of FIG. C1, data for systems featuring this sort of “decoupling” [22] appear as straight line of slope α. In the case of solutions of strong mineral acid in aqueous protic solvents this decoupling appears to commence at a higher fluidity than in the case of silver ions in liquid halides [39, 40]. In less well-known cases such as mineral acids in glycerol the departure occurs at lower fluidities. There is need for additional empirical information on this decoupling phenomenon. The identification of conditions needed for decoupled proton motion in solvent-free systems is very desirable.
The ability of solutions to carry current measured in Sem−1, increases with increasing ion concentration from the low and often immeasurable values of the pure solvent. However it always peaks at concentrations of the order 1M (vs. ˜5M for aqueous)(2) because the electrostatic interaction between the ions of opposite charge moderated by the dielectric constant of the solvent causes a counterbalancing decrease in the individual ionic mobilities. For this reason it is generally not expected that pure salts can be excellent conductors unless the temperature is raised to high values. The inventors show that this expectation is not valid and identify conditions under which the conductivity of solvent-free ionic liquids can be raised to aqueous solution levels.